1. Field of the Invention
The present invention relates generally to the image projection apparatuses manufactured by applying the Micro Electro Mechanical Systems (MEMs) technology. More particularly, this invention relates to an image projection apparatus implemented with a mirror device disposed on a device substrate and packaged in a package substrate made of a transparent silicon material with a metallic thermal transfer path connected to the device substrate and a cover glass covering the package substrate with the distance between the mirror and the bottom surface of the cover glass is larger than the focal length of the projection lens.
2. Description of the Related Art
Design and development of image display apparatuses with high resolutions such as a full high-definition (Full HD: 1920 by 1080 pixels) are in great demand recently. For image projection apparatuses implemented with a mirror device to function as the spatial light modulator (SLM), the demand of high resolution image display imposes a more stringent design and manufacturing requirements on the mirror device, particularly to a mirror device implemented with two-dimensional arrays of micromirrors.
A common mirror device used for a Full-HD (High definition) display apparatus has the diagonal size of 24.13 mm (0.95 inches), with a mirror pitch of 11 μm. An eXtended Graphics Array (XGA)-sized mirror device has the diagonal size of 17.78 mm (0.7 inches) wherein the mirror array has a mirror pitch of 14 μm. In order to provide a projection apparatus to achieve higher resolution it is desirable to shrink the mirror size of the mirror element of a mirror device. Furthermore, it is desirable to provide a package for protecting the mirror device. The package is configured to cover the mirror device with an intermediate member on the device substrate of the mirror device to support a cover glass.
Such a configuration of placing an intermediate member on the device substrate, however, needs to secure a region for placing the intermediate member of the device substrate. Consequently, the device substrate becomes as large as the region necessary for placing the intermediate member on the device substrate. This in turn limits the number of units of device substrates that are cut out of one semiconductor wafer, increasing the cost of the device substrate.
Furthermore, if the number of units of device substrate that are cut out of a semiconductor wafer is limited, the ratio of defective units to the number of usable units is increased making it difficult to improve the yield in production. Meanwhile, conventional methods use a package in which a mirror device is placed on a ceramic substrate and the mirror device is covered with a metallic cover accompanied by a cover glass. The package uses a metallic cover that has an advantage in heat dissipation by radiating and transferring the heat generated from the light irradiated on the mirror device and from the operation of the mirror device thus preventing a temperature rise inside the package.
The processes for manufacturing a package with metallic cover are however more complex. Furthermore, there is another problem with the metallic cover that usually have a large area size and the shape and size of the metallic cover can have a significant change with the variations of temperature due to the coefficient of thermal expansion of the metallic cover. There are additional technical problems in designing and manufacturing the package associated with the miniaturization of the mirror device. For example, the size of a mirror array with approximately two million pixels used for a full high definition (FULL-HD) is currently about 24.13 mm (0.95 inches). In order to reduce the mirror array to a size of 10.16 mm to 22.098 mm—(0.4 to 0.87 inches), a package needs to be designed that by taking into consideration of the issues such as the radiation of heat generated by the mirror device, the reduction of floating capacitance, light shielding for preventing extraneous incident light and a change in the package forms due to temperature, in addition to a miniaturization of the mirror. As discussed above, for the purpose of containing and protecting the mirror device, it is critical to provide a package with improved heat conduction configuration to effectively dissipate the heat generated inside the package.